"""
Caching downloader for planetary science datasets.

Downloads are stored in sol_data/.cache/ and reused on subsequent runs.
Supports resume for large files and optional SHA-256 verification.
"""

import hashlib
import sys
import urllib.request
from pathlib import Path

CACHE_DIR = Path(__file__).resolve().parent / ".cache"


def _progress_hook(block_num, block_size, total_size):
    """Print download progress."""
    downloaded = block_num * block_size
    if total_size > 0:
        pct = min(100.0, downloaded * 100.0 / total_size)
        mb = downloaded / (1024 * 1024)
        total_mb = total_size / (1024 * 1024)
        sys.stdout.write(f"\r    downloading: {mb:.1f}/{total_mb:.1f} MB ({pct:.0f}%)")
    else:
        mb = downloaded / (1024 * 1024)
        sys.stdout.write(f"\r    downloading: {mb:.1f} MB")
    sys.stdout.flush()


def ensure_cached(url: str, filename: str, sha256: str = None) -> Path:
    """
    Download a file if not already cached. Returns path to cached file.

    Parameters
    ----------
    url      : download URL
    filename : local filename within the cache directory
    sha256   : optional hex digest for verification
    """
    CACHE_DIR.mkdir(parents=True, exist_ok=True)
    local_path = CACHE_DIR / filename

    if local_path.exists():
        if sha256:
            actual = _sha256(local_path)
            if actual != sha256:
                print(f"    WARNING: checksum mismatch for {filename}, re-downloading")
                local_path.unlink()
            else:
                return local_path
        else:
            return local_path

    print(f"    fetching {filename} from {url[:80]}...")
    tmp_path = local_path.with_suffix(".tmp")

    try:
        # Many government data servers (USGS, NOAA) require a User-Agent
        opener = urllib.request.build_opener()
        opener.addheaders = [
            ("User-Agent", "SettledReach-PlanetGen/1.0 (terrain pipeline)"),
        ]
        urllib.request.install_opener(opener)
        urllib.request.urlretrieve(url, str(tmp_path), reporthook=_progress_hook)
        print()  # newline after progress
    except Exception as e:
        if tmp_path.exists():
            tmp_path.unlink()
        raise RuntimeError(f"Download failed for {filename}: {e}") from e

    if sha256:
        actual = _sha256(tmp_path)
        if actual != sha256:
            tmp_path.unlink()
            raise RuntimeError(
                f"Checksum mismatch for {filename}: "
                f"expected {sha256[:16]}..., got {actual[:16]}..."
            )

    tmp_path.rename(local_path)
    return local_path


def _sha256(path: Path) -> str:
    h = hashlib.sha256()
    with open(path, "rb") as f:
        for chunk in iter(lambda: f.read(8192), b""):
            h.update(chunk)
    return h.hexdigest()